1. Technical Field
The present invention relates to touch screens, and more particularly, to a touch screen that operates in a capacitive mode with improved noise immunity.
2. Description of Related Art
A schematic of a prior art capacitive touch screen sensor is shown in FIG. 1. A portion 100 of a capacitive touch screen is shown, having a plurality of open loop X-lines X1-X4 (extending parallel to each other in an x-axis direction), and a plurality of open loop Y-lines Y1-Y6 (extending parallel to each other in a y-axis direction). A portion 102 of the touch screen is further highlighted where, for example, the second X-axis line X2 crosses the fifth Y-axis line Y5. A cross-capacitance CC exists between these two lines (as well as at each intersection location 102 where one of the X-axis lines crosses one of the Y-axis lines). A capacitive touch screen sensor device measures the cross-capacitance at the intersection points between each of the X-axis lines and the Y-axis lines. When a finger presses at or near an intersection between two lines, the cross-capacitance at that intersection changes. A charge amplifier is used to quantify the charge transferred by the cross-capacitance, and the value can be digitized and further processed to make a touch detection as well as to locate that detected touch.
Referring now to FIG. 2, a charge amplifier 200 is shown for use in conjunction with a capacitive touch screen sensor like that shown in FIG. 1. Charge amplifier 200 includes a differential amplifier or operational amplifier 202 having a positive input for receiving a VREF reference voltage. An input signal 204 represents the signal input supplied by the user (for example, through a touching of the sensor at or near the location where the lines Xn and Yn cross), and capacitor CC represents the cross-capacitance at that location as shown in FIG. 1. A feedback impedance including resistor Rx and capacitor Cx is coupled between the output 208 and the negative input of amplifier 202. In operation, a rising edge input signal 204 is applied to a representative X-axis line Xn (referred to as the transmit line). The cross-capacitance (CC) transfers charge to the corresponding Y-axis line Yn (referred to as the receive line). The charge amplifier 202 amplifies the charge and stores it across amplifier 202 while the capacitance Cx and Rx discharge the capacitor slowly. The width of a voltage glitch (“t”) in the signal 206 at the output 208 of the amplifier 202 is proportional to the measured cross-capacitance with some degree of non-linearity.
Other circuit techniques for measuring cross-capacitance are well known to those skilled in the art.
With reference to FIG. 3A, a prior art capacitive touch screen sensor includes Y-axis lines Y1-Y11 (reference 110; lines extending parallel to each other in the y-axis direction) and X-axis lines X1-X8 (reference 120; lines extending parallel to each other in the x-axis direction). An example touch 130 is illustrated. As described above, the capacitance of the sensor array (specifically at the intersection points at or near the touch) changes as a result of the presence of the human finger at the touch 130. The charge amplifier 20 of FIG. 2 can be used as a measurement circuit with respect to each location where the lines Xn and Yn cross to measure the cross-capacitance (CC). The change in measured cross-capacitance as a result of the presence of the human finger at the touch 130 is indicated by the capacitance histograms 140 and 150 (with histogram 140 indicating the measured change in cross-capacitance along the X axis and the histogram 150 indicating the measured change in cross-capacitance along the Y axis). The values of the measured capacitance can be processed so as to identify an estimate of the touched location.
FIG. 3B illustrates the combination of the capacitance histograms 140 and 150 of FIG. 3A illustrated in three dimensions as a three dimensional cross-capacitance histogram representative of the touch 130.
FIG. 4A is similar to FIG. 3A except that noise has been introduced. As is known to those skilled in the art, the noise in a capacitive touch screen sensor potentially affects the whole receive line. So, where the X-axis lines Xn are the transmit lines and the Y-axis lines Yn are the receive lines, the noise would be detected along the Y-axis lines and show up in the histogram 150 indicating the measured capacitance along the Y axis. This is indicated in FIG. 4 at reference 155 by an additional peak in the histogram 150. If the measured capacitance associated with the additional peak at reference 155 exceeds a detection threshold, then this noise might mistakenly present an additional touch 160 detection which would be a “false” touch detection made in addition to the correct touch detection associated with touch 130.
FIG. 4B illustrates the combination of the capacitance histograms 140 and 150 of FIG. 4A illustrated in three dimensions as a three dimensional cross-capacitance histogram representative of the touch 130 and the false touch 160 due to the presence of noise on the receive lines.
Referring now to FIG. 5, a capacitive touch screen system 500 is shown. The touch screen system 500 includes a touch screen sensor 510, which comprises a plurality of X-axis lines Xn (extending parallel to each other in the x-axis direction) and a plurality Y-axis lines Yn (extending parallel to each in the y-axis direction). The touch screen system 500 also includes a first switch matrix 506, comprising a receive (RX) switch matrix, having a plurality of inputs respectively coupled to an end of each of the Y-axis lines Yn. The output of the first switch matrix 506 is coupled to the input of a capacitive measurement circuit 520, such as the circuit shown in FIG. 2, and the output of the capacitive measurement circuit 520 is coupled to the input of a processing circuit 516. The touch screen system 500 further includes a second switch matrix 508, comprising a transmit (TX) switch matrix, having a plurality of outputs respectively coupled to an end of each of the X-axis lines Xn. An input to the second switch matrix 508 is coupled to the output of a charge circuit 510.
To measure the cross-capacitance in a capacitive operating mode of the touch screen system 500, the first switch matrix 506 and second switch matrix 508 are configured to make selections of the X-axis lines and the Y-axis lines. The charge circuit 510 applies a charge to one X-axis line (operating as a transmit line) selected by the second switch matrix 508. Charge is transferred from that X-axis line to each of the Y-axis lines Yn (operating as receive lines), that transferred charge being modified by the presence/absence of a touch being made to the sensor. The measurement circuit 520 is then sequentially connected by the first switch matrix 506 to each of the Y-axis lines Yn and a measurement of capacitance is made for each line at the point of intersection between the selected X-axis line and each of the Y-axis lines. The second switch matrix 508 then selects another X-axis line and the process of measuring capacitance for each of the Y-axis lines Yn is performed again. This repeats until the second switch matrix 508 has selected each of the X-axis lines Xn, and a measurement of capacitance has been made as to each point of intersection between an X-axis line and a Y-axis line. The result of the foregoing process is, for example, a data matrix populated by the measured capacitances at each intersection point. The processing circuit 516 may then process the measured capacitances in the data matrix by applying an appropriate threshold value so as to identify instances of a touch. As discussed above, however, noise can adversely affect the capacitive measurements made along an entire receive line (in this case, an entire y-axis line Yn) even if the noise is injected in certain nodes. The noise along the receive line may be detected as a capacitance change by the measurement circuit 520 and evaluated by the processing circuit 516 as a false touch where the measured capacitance at intersection points along the receive line exceeds the touch threshold.
There is a need in the art to address the foregoing problem and provide for improved noise immunity with respect to the operation of capacitive touch screen sensors.